Re: [patch] Fix PR middle-end/56474

[Eric Botcazou <ebotcazou at adacore dot com>]

> Maybe we should detect overflow as if the input and output were signed
> while computing an unsigned result.  As far as I can see int_const_binop_1
> does detect overflow as if operations were signed (it passes 'false' as
> uns to all double-int operations rather than TYPE_UNSIGNED).
> For example sub_with_overflow simply does
> 
>   neg_double (b.low, b.high, &ret.low, &ret.high);
>   add_double (low, high, ret.low, ret.high, &ret.low, &ret.high);
>   *overflow = OVERFLOW_SUM_SIGN (ret.high, b.high, high);
> 
> which I believe is wrong.  Shouldn't it be
> 
>   neg_double (b.low, b.high, &ret.low, &ret.high);
>   HOST_WIDE_INT tem = ret.high;
>   add_double (low, high, ret.low, ret.high, &ret.low, &ret.high);
>   *overflow = OVERFLOW_SUM_SIGN (ret.high, tem, high);
> 
> ?  Because we are computing a + (-b) and thus OVERFLOW_SUM_SIGN
> expects the sign of a and -b, not a and b to verify against the
> sign of ret.

But int_const_binop_1 is called from int_const_binop, so why would we want to 
introduce any overflow for unsigned types other than sizetypes?

> I'm sceptical.  Where do you compute the size expression for variable-sized
> arrays?  I suppose with the testcase in the initial patch I can then inspect
> myself what actually happens?

Sure, but we already went through this in the PR.  It's because of the formula 
used for the length of variable-sized arrays, which needs to handle the case 
of superflat arrays.

-- 
Eric Botcazou